Ethers of polysaccharides, including cellulose ethers, are commonly prepared batchwise by first mixing the polysaccharide with a strong alkali, e.g., an aqueous alkali metal hydroxide in the presence of air, and thereafter reacting the alkali-cellulose with an alkyl chloride, a monochloro carboxylic acid or an unsubstituted vicinal epoxide. Usually the alkyl chloride contains from 1 to 2 C atoms, the chloroacid contains from 2 to about 4 C atoms and the epoxide can contain from 2 to about 4 C atoms and it can be halogenated, e.g. epichlorohydrin. The alkyl chlorides react to form alkyl ethers, the chloroacids form carboxyalkyl ethers and the epoxides form hydroxyalkyl ethers. Mixtures of an alkyl chloride and a vicinal epoxide form derivatives having both alkyl ether and hydroxyalkyl ether groups on the polysaccharide. Similarly, a mixture of an alkyl chloride and a chloro carboxylic acid will result in the formation of both alkyl ether and carboxyalkyl ether groups on the polysaccharide. If desired, all three types of alkylating agents can be mixed for reaction with the polysaccharide to form derivatives having alkyl, carboxyalkyl and hydroxyalkyl ether groups on the polysaccharide. The above etherifying ingredients can be used in this invention.
The reaction can be effected in a dry process wherein the alkali metal cellulose, after preparation, is reacted with liquid, non-aqueous reactants mentioned above in quantities insufficient to make a slurry, or it can be effected in the presence of small quantities of water, with or without the presence of an inert diluent.
Since the polysaccharide ethers are made by successive reactions in which an alkali metal polysaccharide is first prepared and thereafter reacted with an etherifying agent for an extended period of time, continuous processes for making polysaccharide ethers have not been given much attention. One such continuous process is described in U.S. Pat. No. 3,544,556, in which an alkali cellulose is prepared in a intermeshing double screw press according to U.S. Pat. No. 3,615,254 and is then fed into one end of a reactor equipped with a screw conveyor and a condenser. Liquid methyl chloride and, if desired, an alkylene oxide, is fed into the reactor near the entry of the alkali cellulose. The quantity of the liquid is 5 to 10 times the weight of the initial cellulosic component. The reactor is jacketed for control of temperature and it also has a reflux system for condensing vapors and returning them to the reactor. The screw conveyor moves the mixture through the reactor to a pressure maintaining exit port opposite the entry. The mixture is washed under pressure with hot water while simultaneously flashing off some of the more volatile reactants. Pressure is reduced in another vessel and unreacted etherifying agents are evaporated, condensed and recycled to the reactor. In this process temperature control throughout the reaction mass is almost impossible to attain at accelerated reaction rates. In addition, reflux condensers are subject to plugging.